The first representative of the genus Rhopalophthalmus in European waters, Rhopalophthalmus tartessicus sp. nov., is described from specimens sampled in the Guadalquivir estuary (southwestern Spain). At first identified as the Algerian species Rhopalophthalmus mediterraneus Nouvel, 1960, the new mysid species can be distinguished from its closest relative by its higher number of articles on the carpopropodus of the third to seventh thoracic endopods, by the well hook-shaped eighth thoracic endopod of the male, by the slender eighth thoracic endopod of the female, and by the smaller antennal scale and telson. Its geographical distribution appears restricted to estuarine habitats in southwestern Spain. Its swimming behaviour, with the ventral side facing upwards, is unusual in mysids. An updated identification key for the 19 known species in the genus, including information on the respective geographical distribution and habitat, is presented in the above paper.

Diagnosis: Carapace with two dorsal median nodules. Eyes not extending beyond distal margin of second article of antennular peduncle. Antennal scale 4.4-5.0 times as long as broad and slightly longer than antennular peduncle. Antennal sympod armed with two long and equal inner teeth and a shorter ventral one. Carpopropodus of seventh thoracic endopod with 9-11 articles. Apical spines of telson armed with non-flattened spinules.

Description: Adult male: Carapace short posteriorly, exposing the last three thoracic somites. Two small median dorsal nodules: first one immediately posterior to cervical sulcus, second one in middle of posterior margin of carapace (Fig. 2). Carapace tegument rough, due to presence of minuscule scattered scales. Anterior margin slightly convex between the two post-orbital teeth, without trace of rostral projection. Post-orbital teeth very well marked. Antero-lateral angles pointed, sharper than post-orbital teeth, but not extending beyond them. Cheeks straight (Fig. 3E and F). First article of antennular peduncle slightly longer than or equal to combined length of following two articles; more robust in males than in females. Outer margin bearing seven (one bigger than others) short plumose setae about one-fourth along distal length, distal angle armed with three short plumose setae. Dorsal surface with large depression (Fig. 3D) directly beneath eye, fringed with 13 long curved setae along outer rim and with six shorter setae along inner edge. Second article short, with plumose setae at both distal angles. Inner distal margin armed with one robust tooth. Inner margin bearing two coiled, non-plumose setae midway along margin and one longer plumose seta. Distal margin bearing seven plumose setae, slightly projecting forward in middle. Third article about as long as broad, with seven long plumose setae on inner distal angle and seven shorter setae along distal half of inner margin. One seta on outer distal angle. Basal part of outer flagellum hirsute (Fig. 3A).

Antennal scale extending beyond antennular peduncle (Fig. 3E), five times as long as broad. Outer margin straight, naked and terminating in a sharp tooth extending beyond apex of scale. Internal margin parallel to outer margin almost along entire length, armed with 18-20 long plumose setae and five smaller distal setae (Fig. 3B). Apex rounded, with six setae. Inner distal angle of antennal sympod armed with two long, strong, equal, smooth teeth and a shorter more ventral tooth (Fig. 3C).

First thoracic endopod short, with a well-developed inner lobe, articles robust and densely setose (Fig. 4A). Endopod of second thoracic limb long and robust, the distal two articles armed with barbed spines; one long seta on lateral margin of third article (Fig. 4B). Endopods from third to seventh thoracic limbs increasing in length, carpopropodus with eight articles in third limb, nine articles in fourth limb, 10 articles in fifth and sixth limbs, and 11 in seventh limb.

Segmentation not well-marked in preserved specimens. Four grouped distal setae on each article except the basal one, two inner setae smaller than outer ones, V-shaped; inner small setae with serration (Fig. 4D-F). Eighth thoracic endopod reduced, hook-shaped and composed of three parts but considered as non-segmented (articulations not visible). First proximal part robust and bigger than the two others. Second part short, distally bearing six long setae. Third distal part half maximal length of exopod basal article (L/l=2.1), apex conical and bearing four small setae (Fig. 5A). Pleopods biramous (Fig. 6B). Endopod of first pair less than half length of exopod and reduced to a single article, armed with seven setae along inner margin. Exopod with 12 articles armed with plumose setae. Sympod bearing row of 13 long and plumose setae along inner margin (Fig. 6A). Exopod of second pair with 15 articles, three times as long as endopod. Distal eight articles without marginal setae. Article length progressively increasing towards apex. Distal article bearing one strong barbed seta on inner margin, one smooth seta and one barbed seta at apex. Endopod with 10 articles and large pseudobranchial lobe. Third to fifth pleopods with 11 articles on exopod and endopod, both similar in length.

Uropod setose all around, both rami divided by transverse articulation, at about three-quarters and two-quarters length from base in endopod and exopod, respectively. Endopod distinctly tapered, armed with strong spine near midlength on inner margin, outer margin with short setae interspersed with longer setae in distal half. Exopod slightly longer than endopod, apex blunt (Fig. 7A).

Adult female: Endopod of eighth thoracic limb tapered, 1.3 times as long as basal article of exopod, constricted at two-thirds length from its base (but without clear articulation), a single long seta at about midlength of inner margin, apex constricted, pointed and bearing a small seta (Fig. 5C). Distal part bent and crumpled in another specimen (Fig. 5B). Pleopods small, in the form of single plates, increasing in size posteriorly. Sympod of first pleopod armed with seven long plumose setae along inner margin, three setae on outer margin, and two apical plumose setae. Second pleopod bearing eight long plumose setae on inner margin, four setae on outer margin, and two long setae at apex (Fig. 6C). Remaining pleopods similar in form to second pair, with 10 long plumose setae along inner margin in third and fourth pleopod, 12 long plumose setae and two proximal shorter ones along inner margin in fifth pleopod, four setae on outer margin in all of them (Fig. 6D).

Remarks: As mentioned above, R. tartessicus sp. nov. shows a close resemblance to R. mediterraneus from the Algerian coast. Unfortunately, the holotype and paratypes of the latter species are no longer available and attempts to get new specimens were unsuccessful. Therefore, morphological comparisons of the two species are limited to adult specimens and exclusively based on Nouvel's (1960) description. As shown in Table 3, adult specimens from the Guadalquivir estuary can be distinguished from adult stages of R. mediterraneus by several morphological features: distinctly lower length/width ratio of antennal scale, higher number of articles on the carpopropodus of thoracic endopods (mainly on the posterior ones), well hook-shaped eighth thoracic endopod in the male, more slender eighth thoracic endopod in the female (more or less equal to inner margin of exopod basal article), and distinctly lower length/width ratio of telson.

R. tartessicus sp. nov. appears restricted to estuarine waters of southwestern Spain: Guadalquivir estuary, tidal channels and small estuaries of the Bay of Cadiz, and Ria de Huelva.

Depth distribution: 0-10 m, coastal and estuarine species

Ecology:

In the Guadalquivir river, R. tartessicus sp. nov. is mainly concentrated downstream, in the lower 20 km of the estuary. This population shows considerable abundance fluctuations during its annual life cycle, with a minimum observed in winter and maxima in spring and summer. The highest abundances were generally observed at salinities between 10% and 30% and temperatures higher than 15 C. This population also shows a sexual segregation of individuals along the salinity gradient: females were clearly dominant at the most upstream station (32 km from the estuary mouth), while males were significantly more abundant in the lower estuary, as were juveniles and sub-adults. Brooding females were present in all the monthly samples carried out in the estuary during several annual cycles, indicating that R. tartessicus is reproductively ctive during the entire year.